The Bush administration's most senior advisers on Iraq, the commander of US forces, General David Petraeus, and the ambassador to Baghdad, Ryan Crocker, will launch a new drive today to defer any exit of troops until April 2008 amid growing doubts about their credibility in Congress and among the public.

In two days of testimony before Congress, Gen Petraeus and Mr Crocker will make the case for the White House that America should maintain the current strategy and force levels in Iraq.

The appearance of Gen Petraeus has drawn comparisons with General William Westmoreland's bullish assessment in 1967 of progress in the Vietnam war, a note underlined by Democratic senators yesterday who said they doubted that the general's testimony would be free of influence from the Bush administration.

Dick Durbin, the second ranking Democrat in the Senate, was even more blunt. He told reporters: "By carefully manipulating the statistics the Bush Petraeus report will try and persuade us that the strategy is working."

The scepticism about Gen Petraeus's candour was reflected in a new Washington Post/ABC News poll yesterday revealing that more than half of Americans believe that Gen Petraeus will attempt to sugar-coat the situation in Iraq. Some 66% believe that Mr Bush will resist any exit from Iraq, no matter what the general says in his testimony to Congress.

The same poll found the proportion of Americans believing the surge is working was under 30%. It also found a new high of 58% of Americans supporting a reduction in the US military presence in Iraq.

Meanwhile, the grassroots liberal organisation MoveOn.org said it would publish a full-page ad today in the New York Times headlined: "General Betray Us? Cooking the books for the White House."

The general has suggested in a series of interviews over the last few days that he opposed a withdrawal of US forces before the spring when the current troop deployments come to an end.

He has also signalled his intention to shift the focus from the failure of the central aim of the surge. The additional troops were sent to Iraq last January to give breathing space to the government of Nuri al-Maliki so it could effect political reconciliation. With no sign of progress from the central Iraqi government, Gen Petraeus will try to direct the public's attention to the marginal gains made in local security in Anbar province.

Democratic senators took issue yesterday with the general's attempts to recast the mission. "The fact of the matter is that this idea of the security gains we have made have had no impact on the underlying security dynamic, none whatsoever," senator Joe Biden, the head of the foreign affairs committee and a candidate for the presidential election, told NBC television.

Mr Crocker has given little advance indication of what he will say in his testimony, but the Associated Press reported at the weekend that he would support Gen Petraeus's position that the troops surge is the only way to stop Iraq descending into further chaos. A career diplomat with numerous tours in the Middle East, Mr Crocker opposed the US invasion of Iraq, but has since argued that it would be dangerous for Washington to abandon the fragile Maliki government.

Mr Crocker is also expected to discuss the continuing problems of corruption and the difficulties of drafting crucial laws on sharing oil revenues.

The testimony from Gen Petraeus and Mr Crocker leads off a week of public debate about the progress of the war. The discussion overlaps with commemorations to mark the sixth anniversary of the 9/11 attacks tomorrow. The debate will set the stage for a televised address by Mr Bush, possibly on Thursday night, in which he will explain to the American public how he intends to go forward in an unpopular war.

In a preview of his address, Mr Bush told reporters on his way back from an economic summit in Australia that he was cheered by his lightning visit to Iraq last week. "No question there's still hard work to do, but my resolve is as strong as it's ever been."

A week in the war

The testimony today to the Senate armed services committee by General David Petraeus, the commander of US forces in Iraq, and Ryan Crocker, the US ambassador to Baghdad, launches a week of public debate about George Bush's war strategy. The two men are to give Congress their assessment of the situation in Iraq following the deployment of an additional 30,000 US troops earlier this year. Gen Petraeus and Mr Crocker have a second day of testimony before the armed services committee tomorrow. Gen Petraeus is also scheduled to hold a press conference on Wednesday. The events are a lead-up to President George Bush's report on the progress of the war, which he must deliver by Saturday, September 15. He is expected to address the American people on Thursday night.